Semiconductor devices are widely applied in electronic industry. The semiconductor devices are manufactured or fabricated on semiconductor material usually called semiconductor wafers. In order to form electronic circuitry of the semiconductor devices, the semiconductor wafers undergo multiple masking, etching, copper planting and polishing processes, and so on.
Traditionally, in the polishing process, chemical mechanical polishing (CMP) technology is used to remove unnecessary copper layers on the semiconductor wafers. A CMP apparatus includes a rotatable table, a polishing pad disposed on the table, a wafer carrier head for gripping the wafer which needs to be polished, and a slurry feeder providing slurry between the wafer and the polishing pad. A downward press force is acted on the wafer carrier head to press the wafer against the polishing pad, which enforces the wafer to rotate relatively to the polishing pad. Then, the wafer is polished.
However, in order to continually shrink the feature dimension of the semiconductor devices, low K dielectric material or air gap structure is applied in the semiconductor devices. Nevertheless both of the low K dielectric material and the air gap structure have a weak mechanical property, so the downward press force acted on the wafer carrier head in the CMP process will damage the low K dielectric material and further damage the semiconductor devices.
For solving the above problem, stress-free polishing (SFP) technology is provided and suitable for manufacturing tiny semiconductor devices. The stress-free polishing technology is based on the electrochemical polishing mechanism to remove the unnecessary copper layers without mechanical force, avoiding damaging low K dielectric layers on the semiconductor wafers. The quality of the semiconductor devices is improved. A SFP apparatus includes a mechanical motion and control system, an electrolyte deliver system, an electricity supply and control system. In the SFP process, chemical liquid is used as the electrolyte and ejected on a surface of the copper layer which needs to be polished by a nozzle.
However, a common nozzle has a serious shortcoming When the nozzle also used as an electrode is used for polishing the wafer, bubbles are easily generated in the nozzle and ejected on the wafer together with the electrolyte, which results in the poor roughness and defects on the surface of the wafer.
Referring to FIG. 6, FIG. 6 is a partial enlarged view of the surface of the wafer after the wafer is polished by using the nozzle. As can be seen from the drawing, there are two concave holes on the surface of the wafer. The two concave holes are generated by the bubbles. Referring to FIG. 7, FIG. 7 is a profile diagram of the surface of the wafer measured by profilometry. The diagram shows a greater wave crest and a greater wave trough thereon. The greater wave crest represents an area covered by the bubbles on the wafer. The greater wave trough represents an area of the concave hole. During the polishing process, the bubbles blocks the electrolyte directly contacting with the surface of the wafer, which causes the area covered by the bubbles cannot be polished. At the same time, the charge at the area covered by the bubbles isn't consumed and shifts to an adjacent area, causing the adjacent area to be polished overly to form the concave hole. The concave hole brings a detrimental impact on the property of the semiconductor device.
Otherwise, the electrolyte distribution range and shape on the surface of the wafer cannot be controlled well, which affects the removal rate and removal uniformity of the copper layer, and also doesn't satisfy different requirements of the polishing process.